In recent years, as a measure for producing a resin molded article for use in automotive parts, home electronics, etc., a molding member has been increasingly employed which forms a resin molding material into a multilayer structure in a mold simultaneously with molding so as to improve the aesthetic properties and diversity of the resin molded article or omit processes of a molding process for reducing cost.
For example, in order to improve the aesthetic properties and diversity of a molded article, a molding method for forming a structure in which a resin layer that is soft, a colored layer of high quality, or a foamed layer is laminated on the surface of the molded article has been proposed. According to the molding method, a first layer resin molding material is injected and charged in a cavity, and then molding is performed by enlarging a cavity capacity when a second layer resin molding material is injected and charged. For a mold clamping mechanism of an injection molding device for use in this molding method, a direct pressure type mold clamping mechanism provided with a hydraulic mold clamping cylinder with which the enlargement of a cavity capacity is relatively easily controlled has been employed (Patent Document 1).
In this molding method, the enlargement amount of a cavity capacity has an influence on the thickness, dimensions, etc., of a molded article and is an important factor. Thus, the enlargement amount of a cavity capacity is required to be controlled with high precision and high reproducibility.
However, according to such a molding method employing a conventional direct pressure type mold clamping mechanism, control precision of a hydraulic cylinder fluctuates due to compressibility or changes in the viscosity due to temperature changes of hydraulic oil. Moreover, due to a long cylinder stroke, control responsibility is low. There has been a problem that, since a ratio of the control amount of cylinder stroke to the movement amount of mold is 1:1, a molded article with a highly precise thickness cannot be obtained due to low position control properties when multilayer injection molding is performed.
In contrast, a method of controlling a mold platen position of a toggle type mold clamping unit has been disclosed which increases control precision by controlling moving backward of a movable platen by a cross head whose movement amount is larger than the moving amount of the movable platen. According to this method, the interval between the movable platen and a stationary platen can be held constant with high precision (Patent Document 2).
However, according to a conventional molding method including enlargement of a cavity capacity by bending a toggle mechanism using a toggle type mold clamping system, there still are the following problems. More specifically, with a toggle mechanism in a bending state when a space for charging a second layer resin molding material is secured by driving the toggle mechanism for a mold opening to enlarge a cavity capacity after the second layer resin molding material is injected and charged, holding force cannot be obtained which is necessary and sufficient for holding the movable platen position when a charging pressure of the second layer resin acts on the movable mold surface. Therefore, there has been a problem that, when a resin charging pressure acts on the movable mold surface, the movable platen moves backward to further enlarge a cavity capacity, resulting in failure to maintain the thickness precision of a molded article.
Patent Document 3 is mentioned as another prior art reference.    Patent Document 1: JP-A-1979/086550    Patent Document 2: JP-A-1996/309779    Patent Document 3: US/5702810